I'm about to start the design of a motorised loft hatch door opener.
The door in question is about 1m2, made of MDF and opens into the loft space.
My plan is to mount a motor and a spool of string/rope to lift and lower the door.
A wiper motor was recommended as a cheap high torque motor that should be able to lift the quite substantial weight of the door.
My idea is to have the motor turn for x seconds (pre-timed/calculated/tested) but also have a magnet and hall effect sensor in case it isn't open enough and to prevent it opening too far and causing damage.
What I don't know is the best way to control the direction of the motor.
The parts I currently have that I think could be made to do the job in different ways are
4 channel relay
TIP120 transistors
I see the 4 channel relay to be the most simple way to switch polarity/direction of the motor, with the +ve/-ve being wired opposite across pairs of relays. No speed control but very simple and could still be suitable to "nudge" the motor for smaller movements.
TIP120 transistors, not totally sure how to wire these up to enable a change in direction. Should allow for speed control with PWM.
Are either of these reasonable options? Or do I really need a H-Bridge of some description? If so, what spec?
I would think that to be a relatively high current draw situation, not needing PWM control.
I'd go with the relay to control the motor supply. The TIP120 will get really hot and tend to burn out. And not support an H-Bridge.
Unfortunately that cabling does not show the necessary freewheel diodes you will have to use.
Do you need further information about where to place and how to wire the freewheel diodes in your circuit?
I've never used a H-Bridge before so that is something I didn't know, which is great news because I also have a 2 channel relay doing very little right now.
Thanks for the very useful link.
More information about where I'd need to put the freewheel diodes would be great.
The plan is to use a 433 MHz receiver on the arduino and have the system totally in the loft, then use a 433 MHz remote to control it.
ant_thomas:
More information about where I'd need to put the freewheel diodes would be great.
Here is a schematic with relays and freewheel diodes:
Those diodes are always to be connected in back direction across the opening contacts.
Be carefull not to create a short circuit!
Freewheel diodes have ALWAYS to be used when switching "inductive DC loads".
But the 4 diodes as shown above for switching the motor in H-bridge wiring is not enough.
Also the coil of the relay itself is an "inductive DC load". If you'd use a "relay module", then there would be the required freewheel diodes for the relay coil on the module. But if you are using a "naked" relay and a transistor to switch it, you will have to use an additional freeewheel diode at the switched side of the relay coil, too.
Of course, you will have to have a power supply for the wiper motor, so a source AC power will be necessary.
I stopped by a Habitat for Humanity thrift store this afternoon and saw the motor box part of a garage door opener for sale. This has a high torque motor and is reversible. Could you find something like that for your door opener?
forget the wiper motor. look at power window motors. a wiper is usually a sealed system.
also, use counterweight to make the door easier to open. I would think that there are quite a few on here that would not want to stand under some device they built.
I think for a simple jroject, you could use a line rated switch, on at the top, one at the bottom. when the switch is not in contact, it would allow power to run though it. then, have a simple up-down switch. when you lift it up, the top switch allows power, the motor will start to run to lift the hatch.
once it contacts the switch, the circuit is open and the unit cannot rotate in that direction any more.
very simple.
if you use a momentary, 3 postion, center off switch you would have to press and hold.
that way, you would only power the power supply while you are holding the switch.
jurs:
Here is a schematic with relays and freewheel diodes:
Those diodes are always to be connected in back direction across the opening contacts.
Be carefull not to create a short circuit!
Freewheel diodes have ALWAYS to be used when switching "inductive DC loads".
But the 4 diodes as shown above for switching the motor in H-bridge wiring is not enough.
Thanks.
Relay module already has diodes so that is fine.
So I just need 4 diodes and to connect them as shown across the relay module screw terminals (being careful to get them the correct way round)?
Apart from actually connecting the diodes and motor (when it arrives) I've got the breadboard prototype working seemingly correctly, switching between directions with a 433 MHz remote. I've also added two hall effect sensors as a fail safe to hopefully prevent over opening/closing.
I'm quite happy with the speed and motion, nothing too fast but also not too slow. Waiting for some magnets to arrive to improve the hall effect sensor part.
I've got it running off a 12V 10A supply which I would hope is more than upto the job.
